Muting circuit for analog filters in radio frequency identification (rfid) systems

ABSTRACT

An apparatus, which allows a radio frequency identification (RFID) reader to recover quickly from transient input to its receiving subsystem when transitioning from writing to an RFID transponder to reading its response. In particular, this apparatus is comprised of muting circuits, which both attenuate transients in its receiving subsystem while writing to a transponder, and help the receiving subsystem settle quickly after experiencing such transients.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 16/857,046, filed Apr. 23, 2020, the entirecontents of which is incorporated herein by reference.

BACKGROUND

This specification relates to Radio Frequency Identification (RFID)systems, and in particular to magnetically-coupled passive RFID systems.

As shown in FIG. 1, passive RFID systems typically include two majorsubassemblies: an RFID reader, and an RFID transponder which is to beread at some distance from the RFID reader. The RFID reader includes anAC voltage source that drives its resonant antenna coil circuit. In thismanner, the RFID reader emits an alternating magnetic field from itsantenna coil, which is weakly magnetically coupled (represented in FIG.1 by a dashed double arrow) to a corresponding antenna coil in thetransponder to be read. Each of these antenna coils is part of acorresponding antenna coil circuit that includes one or more tuningcapacitors to cause the corresponding antenna coil circuit to resonateat a desired frequency.

The transponder obtains its operating power from the RFID reader'semitted magnetic field, and modulates (e.g., using a switch and loadingresistor) the Q factor and/or resonant frequency of its antenna coilcircuit in a pattern corresponding to any information which is to besent from the transponder to the RFID reader. This information commonlyincludes an identification number that uniquely corresponds to theindividual transponder.

Due to the magnetic coupling between the RFID reader and thetransponder, the transponder's modulation appears as variations in theelectrical currents and voltages present in the RFID reader's antennacoil circuit. The RFID reader can then use a receiving subsystem(represented by RX in FIG. 1) to detect and demodulate these variationsin order to retrieve whatever information the transponder sends. Thisreceiving subsystem typically includes an amplitude modulation detectorand one or more filtering and/or gain stages. Maximum reading range istypically achieved when both the RFID reader's antenna coil circuit andthe transponder's antenna coil circuit are tuned to resonate at thefrequency of the RFID reader's AC voltage source, both antenna coils areoriented for optimal magnetic coupling, both antenna coil circuits havethe highest practical Q factors, and the transponder modulates its Qfactor as deeply as practical while still receiving enough power fromthe RFID reader for its circuitry to operate.

The information present in a transponder is typically stored in someform of nonvolatile memory. This memory may include a combination offactory-programmed and/or field-programmable memory locations. Sometransponders may also generate and/or store dynamic information, such asa temperature transducer reading. Typically, the transponder willautomatically send a subset of its stored information when it isactivated by the magnetic field emitted from a nearby RFID reader.

Some transponders are not only readable, but are also writable. Acompatible RFID reader can modulate its own emitted alternating magneticfield (e.g., by turning on and off the AC voltage source driving itsresonant antenna coil circuit) in order to send commands and/or data toone of these transponders. This function is called “writing”, as opposedto the previously described function of “reading”. A device whichperforms both reading and writing functions is usually still genericallyreferred to as a “RFID reader”. Transponder writing may be used forpurposes such as commanding the transponder to send a differentcollection of information than it automatically sends by default, forinitial programming of the transponder at time of manufacture, forprogramming field-programmable memory locations, or for activatingspecial transponder functions.

SUMMARY

When writing to an RFID transponder, it is often necessary for an RFIDreader to send a command to the transponder, and then immediately listenfor a response from the transponder. While the RFID reader is modulatingits emitted magnetic field to send a command to the transponder, thedemodulator portion of the RFID reader sees this modulation as anextremely strong signal, which is much stronger than the signal normallyreceived from the transponder. This strong signal is likely to saturatethe RFID reader's demodulation circuits. Worse yet, filters in thedemodulation circuits may require a significant amount of time (i.e.,relative to the time it takes to send a receive transmissions) torecover from this saturation, and this recovery may not complete in timefor the RFID reader to demodulate successfully the transponder'sresponse. This issue becomes more problematic when higher filterperformance is desired in order to improve the RFID reader's transponderreading range. This situation forces a compromise between the RFIDreader's transponder reading performance vs. its ability to writecommands and data to transponders.

This specification describes a demodulator filter chain, which has highgain and narrow bandwidth in order to demodulate weak signals fromtransponders at extreme range, yet also can recover quickly from largetransient inputs it will see while an RFID reader modulates its emittedmagnetic field to write commands and data to a transponder.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in circuitry for communicating with aradio frequency identification (RFID) transponder. The circuitryincludes an antenna circuit; and a receiving subsystem coupled with theantenna circuit. The receiving subsystem includes: a detector circuitconfigured to detect voltages present in the antenna circuit, a filterstage comprising a feedback loop, the filter stage configured to outputvariations of the detected voltages that are caused by transmissions toand from the RFID transponder, a microcontroller configured todemodulate the output variations as digital data received from the RFIDtransponder as part of the communications, and a first switch disposedbetween the detector circuit and the filter stage to decouple the filterstage from the detector circuit when the first switch is closed, andcouple the filter stage with the detector circuit when the first switchis open. The microcontroller is configured to maintain the first switchclosed during the transmissions to the RFID transponder, and open thefirst switch after a first time interval since the end of thetransmissions to the RFID transponder, the first time intervalcorresponding to a settling time of the detector circuit.

The foregoing and other embodiments can each optionally include one ormore of the following features, alone or in combination. In particular,one embodiment includes all the following features in combination.

In some implementations, the first switch includes a pair of cascadedtransistors.

The microcontroller can be configured to (i) monitor, after the end oftransmissions to the RFID transponder, an input of the filter stage todetermine whether the detector circuit has settled, and (ii) establish alength of the first time interval based on a result of the determinationwhether the detector circuit has settled.

The receiving subsystem can further include a second switch disposed aspart of the feedback loop of the filter stage to decrease resistance ofthe feedback loop when the second switch is closed, and to increase theresistance of the feedback loop when the second switch is open, themicrocontroller can be configured to maintain the second switch closedduring the transmissions to the RFID transponder, and open the secondswitch after a second time interval since the end of the first timeinterval, the second time interval corresponding to a settling time ofthe filter stage.

The filter stage can include an operational amplifier, and the secondswitch is connected to short the feedback loop of the operationalamplifier, when the second switch is closed. The second switch caninclude a pair of cascaded transistors. The microcontroller can beconfigured to monitor, after the end of transmissions to the RFIDtransponder, an input of the filter stage to determine whether thedetector circuit has settled, and establish a length of the first timeinterval based on a result of the determination whether the detectorcircuit has settled, and monitor, after the end of the first interval,an output of the filter stage to determine whether the filter stage hassettled, and establish a length of the second time interval based on aresult of the determination whether the filter stage has settled.

In some embodiments, the filter stage is a first filter stage and thereceiving subsystem further includes: a second filter stage disposedbetween the first filter stage and the microcontroller, and a thirdswitch disposed as part of the feedback loop of the second filter stage,wherein the microcontroller is configured to open the third switch aftera third time interval since the end of the second time interval, thethird time interval corresponding to a settling time of the secondfilter stage. The second filter stage can include an operationalamplifier, and the third switch is connected to short the feedback loopof the operational amplifier, when the third switch is closed. The thirdswitch can include a pair of cascaded transistors. In some embodiments,the microcontroller is configured to monitor, after the end oftransmissions to the RFID transponder, an input of the filter stage todetermine whether the detector circuit has settled, and establish alength of the first time interval based on a result of the determinationwhether the detector circuit has settled, and monitor, after the end ofthe first interval, an output of the first filter stage to determinewhether the first filter stage has settled, and establish a length ofthe second time interval based on a result of the determination whetherthe first filter stage has settled, and monitor, after the end of thesecond interval, an output of the second filter stage to determinewhether the second filter stage has settled, and establish a length ofthe third time interval based on a result of the determination whetherthe second filter stage has settled.

In general, another innovative aspect of the subject matter described inthis specification can be embodied in a method for communicating with aradio frequency identification (RFID) transponder. The method includes:transmitting an RFID reader RF signal from an RFID reader; receiving, atan antenna circuit of the RFID reader, a transponder RF signaltransmitted by the RFID transponder in response to the RFID reader RFsignal; detecting, during the transmitting and receiving and using adetector circuit of the RFID reader, voltages present in the antennacircuit; after a first time interval since an end of transmitting theRFID reader RF signal, coupling a filter stage to the detector circuit,the filter stage being decoupled from the detector circuit during thetransmitting; filtering, using the filter stage upon coupling the filterstage to the detector circuit, the detected voltages in the antennacircuit to provide output variations; and demodulating the outputvariations to retrieve digital data received from the RFID transpondertransmitted in the transponder RF signal.

The foregoing and other embodiments can each optionally include one ormore of the following features, alone or in combination. In particular,one embodiment includes all the following features in combination.

For example, the first time interval can correspond to a settling timeof the detector circuit.

The coupling and decoupling of the filter stage can be performed with aswitch between the filter stage and the detector circuit. The switch canbe closed during transmitting the RFID reader RF signal. The switch canbe opened after the first time interval.

The method can include varying a resistance of a feedback loop of thefilter stage to decrease resistance of the feedback loop duringtransmissions from the RFID reader. The resistance of the feedback loopcan be varied by opening and closing a second switch, the second switchbeing in the feedback loop. In some embodiments, varying the resistanceof the feedback loop includes increasing resistance of the feedback loopafter a second time interval since the end of the first time interval,the second time interval corresponding to a settling time of the filterstage.

The method can include monitoring, after emitting the RFID reader RFsignal, an input of the filter stage to determine whether the detectorcircuit has settled, and establishing a length of the first timeinterval based on a result of the determination whether the detectorcircuit has settled. The method can include monitoring, after the end ofthe first interval, an output of the filter stage to determine whetherthe filter stage has settled, and establish a length of a second timeinterval based on a result of the determination whether the filter stagehas settled. The filter stage can be a first filter stage and can becoupled to a second filter stage, and the method can include monitoring,after the end of the second interval, an output of the second filterstage to determine whether the second filter stage has settled, andestablishing a length of a third time interval based on a result of thedetermination whether the second filter stage has settled.

The RFID reader RF signal can be a signal for writing information to theRFID transponder.

The subject matter described in this specification can be implemented inparticular embodiments to realize one or more of the followingadvantages. Achieving a long maximum reading range naturally leads thedesigner to implement filter elements in the RFID reader's receivingsubsystem which have high gain and high Q factors, and this necessarilyresults in filter elements that require some finite, nonzero time torecover from very large transient inputs. The receiving subsystem of anRFID reader which is capable of read/write operations must be able torecover quickly enough from writing-induced transients to process areply from a coupled transponder, and this requirement typically limitsthe maximum achievable read-only range. The disclosed technologies allowan RFID reader to reduce or eliminate that compromise, by reducing themagnitude of write-induced transients seen by the receiving subsystem.Furthermore, they decrease the recovery time of the RFID reader'sreceiving subsystem filter elements by controlling the progression of awriting-induced transient through cascaded filter elements.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a passive RFID system.

FIG. 2 is a schematic diagram of an RFID reader including a conventionalreceiving subsystem.

FIGS. 3A-3C show waveforms illustrating circuit behavior correspondingto the RFID reader shown in FIG. 2.

FIG. 4 is a schematic diagram of an RFID reader including a receivingsubsystem configured in accordance with the disclosed technologies.

FIGS. 5A-5C show waveforms illustrating circuit behavior correspondingto the RFID reader shown in FIG. 4.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Described in this specification is a demodulator filter chain that hashigh gain and narrow bandwidth in order to demodulate weak signals fromtransponders at extreme range, yet also can recover quickly from largetransient inputs it will see while an RFID reader modulates its emittedmagnetic field to write commands and data to a transponder.

FIG. 1 shows a schematic diagram of an RFID reader 110 and an RFIDtransponder 160. The RFID reader 110 includes an AC voltage source 112,which drives a tuned antenna coil circuit that includes one or morecapacitors 113 and an antenna coil 115 at or near the tuned antenna coilcircuit's natural resonant frequency. Variations of the voltages and/orcurrents present in the RFID reader 110's antenna coil circuit caused bya coupled transponder 160 are detected and demodulated by a receivingsubsystem 120. The RFID transponder 160 includes an antenna coil 165 andcapacitor 163 which together form a tuned antenna coil circuit, as wellas switching element 169 and resistor 167 which allow the RFIDtransponder 160 to modulate the impedance which it presents to the RFIDreader 110. The respective tuned antenna coil circuits of the RFIDreader 110 and the RFID transponder 160 are coupled to each other byloose magnetic coupling 157.

FIG. 2 shows a schematic diagram of a first example of an RFID reader210 including relevant details of its receiving subsystem 220. Here, thereceiving subsystem 220 is implemented in a conventional configuration.The RFID transponder 160's modulation is detected by a detector 230,which includes a bridge rectifier 13, smoothing capacitor 14, anddischarge resistor 15. The AC component of the detected signal iscoupled through a DC blocking capacitor 16 to one active filter stage240 a or a cascade of two or more active filter stages 240 a, 240 b,etc. Each active filter stage 240 a/b includes an operational amplifier18 with suitable passive components in its input and feedback circuitsto provide the suitable transfer characteristics. From there, thedetected and filtered signal passes on to a microcontroller 250 fordemodulation and display of the information (e.g., using an electronicdisplay) sent by the RFID transponder, e.g., 160. The microcontroller250 is further configured to control the RFID reader 210 in general.

FIGS. 3A-3C illustrate some key waveforms within the RFID reader 210.

FIG. 3A shows an example of a carrier keying waveform 322, whichrepresents the RFID reader 210's modulation of its emitted alternatingmagnetic field 157. Here, a low level corresponds to the AC voltagesource 112 being turned off, while a high level corresponds to a voltageoutput by the AC voltage source 112 when it is on. In someimplementations, the RFID reader 210's modulation of its emittedalternating magnetic field 157 is in a frequency range of 100-400 kHz.For example, a frequency of the RFID reader 210's modulation can bef_(C)=134.2 kHz. The modulated interval of the carrier keying waveform325 corresponds to the RFID reader 210 sending a command to the RFIDtransponder, e.g., 160, prior to leaving its carrier steadily on, whilethe RFID reader 210 listens for a response from the RFID transponder,e.g., 160.

FIG. 3B shows an example of the microcontroller 250's input waveform323, which represents the output voltage from the last of the activefilter stages, which in FIG. 2 is the active filter stage 240 b. FIG. 3Cshows the demodulated bits waveform 324 which corresponds to the inputwaveform 323 converted to binary values prior to extraction of clock anddata by the microcontroller 250. Digital data can be extracted from thedemodulated bits waveform 324. In some embodiments, the amplitude andphase of the output waveform that results from the demodulation (see,e.g., FIG. 3C) are stored and processed as floating point values beforefinal decoding.

Referring now to FIGS. 3A-3C, the modulation of the carrier keyingwaveform 325 results in saturation of the active filter stages 240 a,240 b, which can be clearly seen in the saturated interval of thedemodulator input waveform 326. Once the RFID reader 210 stopsmodulating its emitted carrier, gradual recovery of the active filterstages 240 a, 240 b results in filter ringing within the ringinginterval of the demodulator input waveform 327, with corresponding falsetransitions and distorted bits in the corrupted interval of thedemodulated bits waveform 328. Finally, the RFID reader 210 is able toclearly receive a response from the RFID transponder, e.g., 160, asshown in the normal interval of the demodulator input waveform 329, witha corresponding cleanly demodulated waveform in the properly decodedinterval of the demodulated bits waveform 330. Note that the RFIDtransponder, e.g., 160, transmits its response to the RFID reader 210 asa modulation at a frequency ƒ_(T) smaller by a factor of N than thefrequency ƒ_(C) of the RFID reader 210's modulation,

$f_{T} = {\frac{f_{C}}{N}.}$

Here, N can be 2, 8, 10, 32, 64, or other integer values.

In general, the information stored in a transponder can be any type ofdata (e.g., words, numbers, and other alphanumeric strings), storeddigitally. During transmission, the transponder modulates a carriersignal to encode the digital data into an RF waveform. The readerdemodulates the received waveform to extract the digital data,retrieving the information stored in the transponder. Because this datawas stored digitally on the transponder, it is considered to be digitaldata, even if it is not restored to its original digital form during orafter the demodulation process.

FIG. 4 shows a schematic diagram of a second example of an RFID reader410 including relevant details of its receiving subsystem 420. Here, thereceiving subsystem 420 is implemented in accordance with the disclosedtechnologies. In addition to the receiving subsystem 420, the RFIDreader 410 includes components 112, 113, and 115 that were describedabove in connection with the RFID reader 210.

In addition to components 230, 16, and 250 that it has in common withthe receiving subsystem 220, the receiving subsystem 420 includes adetector-muting switch 425, which will be closed while writing to, andopened while reading from, an RFID transponder, e.g., 160. Thedetector-muting switch 425 both attenuates strong signals from thedetector 230 which are present while writing to the RFID transponder,e.g., 160, and allows the state of charge of the DC blocking capacitor16 to settle more quickly after transitioning from transponder writingback to transponder reading. Here, the settling time relates to thetransient response of the capacitor (or other output device) in responseto a sudden variation in input. For example, settling time can refer tothe time elapsed from an ideal instantaneous step input to the time atwhich the capacitor (or other output device) output remains within acertain error band.

In addition to the foregoing components, the RFID reader 420 includes acascade of active filters 440 a, 440 b implemented in accordance withthe disclosed technologies. Each active filter 440 a/b includes anoperational amplifier 18 with suitable passive components in its inputand feedback circuits like the ones of corresponding filter stage 240 a,240 b. Here, a respective filter stage-muting switch 445 a/b isconnected to the feedback network of each active filter stage 440 a/b.The filter stage-muting switch 445 a/b will be closed while writing andopened while reading. A respective filter stage-muting switch 445 a/ballows each filter stage 440 a/b to recover more quickly aftertransitioning from transponder writing back to transponder reading. Themicrocontroller 250 optionally monitors intermediate circuit nodes,e.g., nodes A, B, C, in the cascade of active filters 440 a, 440 b.Optimal recovery time is achieved when all muting switches 425, 445 a,445 b are closed during transponder writing, and then are opened in acarefully timed sequence starting at the detector 230 and proceedingthrough successive active filter stages 440 a, 440 b. Eachswitch-opening event shall be carefully timed to coincide with theprevious detector stage 230's output, or the previous filter stage 440a, 440 b's output, settling to near its quiescent state, thus minimizingthe perturbation of each filter stage 440 a/b. In some implementations,optimum timing for opening each switching element 425, 445 a, 445 b isdetermined analytically through circuit analysis. In someimplementations, the optimum timing for opening each switching element425, 445 a, 445 b is determined empirically through measurement of thecircuit's response to large transient inputs. In implementations likethe one shown in FIG. 4, the optimum timing for opening each switchingelement 425, 445 a, 445 b is determined as part of a closed-loop systemwhere the microcontroller 250 monitors intermediate circuit nodes, e.g.,nodes A, B, C, to determine when to open each switching element 425, 445a, 445 b.

FIGS. 5A-5C illustrate the improved behavior of signals that correspondto the ones shown in FIGS. 3A-3C as they apply to the RFID reader 410.Here, the carrier keying waveform 322 shown in FIG. 5A is the same asthe one shown in FIG. 3A. FIG. 5B shows an example of the microprocessor250's input waveform 523, which represents the output voltage from thelast of the active filter stages, which in FIG. 4 is the active filterstage 440 b. FIG. 5C shows the demodulated bits waveform 524 whichcorresponds to the input waveform 523 converted to binary values priorto extraction of clock and data by the microprocessor 250. Notice thatthe modulated interval of the carrier keying waveform 325, shown in FIG.5A, does not produce saturation of the active filters 440 a, 440 b, asshown in FIGS. 5B-5C by flat portions of the waveforms 523, 524. FIGS.5B-5C also show that the ringing interval of the demodulator inputwaveform 527 is much shorter, with a correspondingly shorter corruptedinterval of the demodulated bits waveform 528, relative to therespective intervals of the waveforms 323, 324. One or more additionalproperly decoded data bits can now be seen in the normal interval of thedemodulator input waveform 529 and the properly decoded interval of thedemodulated bits waveform 530.

In summary, this specification describes a demodulator filter chain,which has high gain and narrow bandwidth in order to demodulate weaksignals from transponders at extreme range, yet also can recover quicklyfrom large transient inputs it will see while an RFID reader modulatesits emitted magnetic field to write commands and data to a transponder.

Embodiments of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, in tangibly-embodied computer software or firmware, incomputer hardware, including the structures disclosed in thisspecification and their structural equivalents, or in combinations ofone or more of them. Embodiments of the subject matter described in thisspecification can be implemented as one or more computer programs, i.e.,one or more modules of computer program instructions encoded on atangible non-transitory storage medium for execution by, or to controlthe operation of, data processing apparatus. The computer storage mediumcan be a machine-readable storage device, a machine-readable storagesubstrate, a random or serial access memory device, or a combination ofone or more of them. Alternatively or in addition, the programinstructions can be encoded on an artificially-generated propagatedsignal, e.g., a machine-generated electrical, optical, orelectromagnetic signal, that is generated to encode information fortransmission to suitable receiver apparatus for execution by a dataprocessing apparatus.

The term “microcontroller” refers to data processing hardware andencompasses all kinds of apparatus, devices, and machines for processingdata, including by way of example a programmable processor. Themicrocontroller can also be, or further include, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit). The microcontroller canoptionally include, in addition to hardware, code that creates anexecution environment for computer programs, e.g., code that constitutesprocessor firmware, a protocol stack, a database management system, anoperating system, or a combination of one or more of them.

A computer program, which may also be referred to or described as aprogram, software, a software application, an app, a module, a softwaremodule, a script, or code, can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages; and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A program may, but neednot, correspond to a file in a file system. A program can be stored in aportion of a file that holds other programs or data, e.g., one or morescripts stored in a markup language document, in a single file dedicatedto the program in question, or in multiple coordinated files, e.g.,files that store one or more modules, sub-programs, or portions of code.A computer program can be deployed to be executed on one computer or onmultiple computers that are located at one site or distributed acrossmultiple sites and interconnected by a data communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable computers executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby special purpose logic circuitry, e.g., an FPGA or an ASIC, or by acombination of special purpose logic circuitry and one or moreprogrammed computers.

Computer-readable media suitable for storing computer programinstructions and data include all forms of non-volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto-optical disks; andCD-ROM and DVD-ROM disks.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a LCD (liquid crystal display) or organiclight emitting diode (OLED) monitor, for displaying information to theuser and a keyboard and a pointing device, e.g., a mouse or a trackball,by which the user can provide input to the computer. Other kinds ofdevices can be used to provide for interaction with a user as well; forexample, feedback provided to the user can be any form of sensoryfeedback, e.g., visual feedback, auditory feedback, or tactile feedback;and input from the user can be received in any form, including acoustic,speech, or tactile input. In addition, a computer can interact with auser by sending documents to and receiving documents from a device thatis used by the user; for example, by sending web pages to a web browseron a user's device in response to requests received from the webbrowser. Also, a computer can interact with a user by sending textmessages or other forms of message to a personal device, e.g., asmartphone that is running a messaging application, and receivingresponsive messages from the user in return.

Other embodiments are in the following claims.

1-23. (canceled)
 24. A method for communicating with a radio frequencyidentification (RFID) transponder using a RFID reader, the RFID readercomprising a detector circuit, a plurality of active filter stages, anda detector-muting switch arranged between the detector circuit and theactive filter stages, the method comprising: transmitting a commandsignal from an RFID reader to the RFID transponder, the transmittingbeing performed while the detector-muting switch is closed to mute asignal from the detector circuit to the active filter stages; aftertransmitting the command signal, opening the detector-muting switch;detecting, with the detector circuit while the detector-muting switch isopen, a response signal transmitted by the RFID transponder in responseto the command signal; filtering the detected response signal using theactive filter stages; and demodulating, at the RFID reader, the filteredresponse signal to provide digital data received from the RFIDtransponder transmitted in the response signal.
 25. The method of claim24, wherein each of the active filter stages have a corresponding filterstage-muting switch, and each of the corresponding filter stage-mutingswitches are closed during the transmitting of the command signal. 26.The method of claim 25, wherein each of the filter stage-muting switchesare opened after transmitting the command signal.
 27. The method ofclaim 26, wherein the filter stage-muting switches are openedsequentially.
 28. The method of claim 27, wherein the detector-mutingswitch is opened prior to opening the filter stage-muting switches. 29.The method of claim 28, wherein the active filter stages are arranged inseries and the filter stage-muting switches of the active filter stagesare opened sequentially starting with the active filter stage closest tothe detector circuit.
 30. The method of claim 29, wherein the opening ofthe filter stage-muting switch of a subsequent of the active filterstages coincides with an output of the prior active filter stage. 31.The method of claim 27, wherein a relative timing of the opening of thefilter stage-muting switches is determined by a microcontroller of theRFID reader.
 32. The method of claim 25, wherein the active filterstages are arranged in series and each include an output device having acorresponding settling time for a transient response to an input signal,wherein an opening of the filter stage-muting switch of a subsequent ofthe active filter stages is timed to coincide with an output of theprior active filter stage.
 33. Circuitry for communicating with a radiofrequency identification (RFID) transponder, the circuitry comprising:an antenna circuit; and a receiving subsystem coupled with the antennacircuit, wherein the receiving subsystem comprises: a detector circuitconfigured to detect voltages present in the antenna circuit, aplurality of active filter stages, the active filter stages configuredto output variations of the detected voltages that are caused bytransmissions from the RFID transponder, a microcontroller configured todemodulate the output variations as digital data received from the RFIDtransponder, and a detector-muting switch between the detector circuitand the active filter stages to decouple the active filter stages fromthe detector circuit when the detector-muting switch is closed, andcouple the active filter stages with the detector circuit when thedetector-muting switch is open, wherein the microcontroller isconfigured to maintain the detector-muting switch closed duringtransmission of a command signal from the circuitry, open thedetector-muting switch after the end of the transmission of the commandsignal.
 34. The circuitry of claim 33, wherein each of the active filterstages comprises a filter stage-muting switch, and the microcontrolleris configured to maintain each of the filter stage-muting switchesclosed during transmission of the command signal and open the filterstage-muting switches after the end of the transmission of the commandsignals.
 35. The circuitry of claim 34, wherein the microcontroller isconfigured to open the filter stage-muting switches sequentially. 36.The circuitry of claim 34, wherein each of the active filter stagescomprises an operational amplifier and the filter stage-muting switchfor each active filter stage is connected to short a feedback loop ofthe corresponding operational amplifier.
 37. The circuitry of claim 34,wherein each filter stage-muting switch comprises a respective pair ofcascaded transistors.
 38. The circuitry of claim 37, wherein themicrocontroller is configured to monitor, after the end of thetransmission of the command signal, an input of the active filter stagesfrom the detector circuit to determine whether the detector circuit hassettled, and establish a time interval before opening thedetector-muting switch based whether the detector circuit has settled.39. The circuitry of claim 38, wherein the microcontroller is furtherconfigured to monitor an output of each of the active filter stages todetermine whether each active filter stage has settled, and establishrespective time intervals for closing the filter stage-muting switcheach active filter stage based on whether the prior active filter stagehas settled.
 40. The circuitry of claim 33, wherein the detector-mutingswitch comprises a pair of cascaded transistors.